To be in the moment means to be aware of the present. What is happening right now? Right now is the only time you have. Yesterday is gone. The future isn't here yet. Therefore, the only thing you have control over is the present moment. What are you going to do with it? Think about what you could of, should of, or would have done or worry about the future.
A great quote I ran across about worry states, "Worry is like sitting in a rocking chair. It gives you something to do but it doesn't get you anywhere." How true! The present is where it's at and it's the only place you can be. Work on today to make tomorrow what you want it to be.